One material becomes dark/black after baking
So, I found the answer at last. Here's for you that have the same problem:
I'm not quite sure why but if you have a full metallic material (perhaps it darkens less if your material is less metallic. Didn't have the time to play with the bake), the diffuse baking will bake the material as black. So to fix this issue disconnect the metallic map/put the metallic value to zero before baking diffuse and voilà the diffuse map will have the real color.